Wellens, 34, lifted the Super League trophy at Old Trafford last month and will play on in to his 20th season for Saints.
He said: “I’m really looking forward to 2015. With a World Club Challenge to look forward to, the new Super League format and a title to defend it is a very exciting time for the club.
“I’m in the very latter stages of my career and with a good crop of young players coming through my focus is on training hard and competing for a place in the team whilst also using my experience to help these young players develop further.
Wellens joined Saints’ academy in 1996, making his first team debut in 1998, and has gone to score 1001 points in 491 appearances, including 230 tries.
Head Coach Keiron Cunningham added: “It’s great that Paul has agreed to extend his contract by another season.
“He proved his versatility in our Championship winning season by playing in many positions and performing at a high level.
“His commitment to the club is superb; he is the ultimate professional and that rubs off on all those around him. He is also keen to learn and improve, setting himself challenges to get better.
“His strong and respected relationship with the players is evident by the fact he wants to help improve Jonny Lomax‘ game at fullback – the position he plays in.
“That is a fantastic and unselfish example to the young players in the squad.”
During his time with the club, Wellens has won two World Club Challenges, five Super League titles and five Challenge Cups as well as being part of the team that has secured the League Leaders’ Shield five times.
He also won the Man of Steel and Harry Sunderland Awards in 2006 and the Lance Todd Trophy in 2007 and 2008.